A study, published Thursday in the journal Current Biology, showed that certain neurons in the brain were responsible for marking the beginning and end of the chunked units of behavior.

Neuroscientists from Massachusetts Institute of Technology (MIT) found neurons located in striatum, a brain region highly involved in habit formation, fired at the outset of a learned routine, went quiet while it was carried out, then fired again once the routine had ended.

This task-bracketing appears to be important for initiating a routine and then notifying the brain once it is complete, said Ann Graybiel, an Institute Professor at MIT, and the senior author of the study.

Once these patterns form, it becomes extremely difficult to break the habit, said Graybiel.

Graybiel and her team set out to determine whether this firing pattern could be conclusively linked with the chunking of habitual behavior.

The researchers trained rats to press two levers in a particular sequence, for example, 1-2-2 or 2-1-2. The rats had to figure out what the correct sequence was, and if they did, they received a chocolate milk reward.

It took several weeks for them to learn the task, and as they became more accurate, the researchers saw the same beginning-and-end firing patterns develop in the striatum.

Because each rat learned a different sequence, the researchers could rule out the possibility that the patterns correspond to the motor input required to perform a particular series of movements.

This offers strong evidence that the firing pattern corresponds specifically to the initiation and termination of a learned routine, the researchers said.

The researchers also discovered a distinct pattern in a set of inhibitory neurons, known as interneurons, in the striatum.

The interneurons would be activated during the time when the rats were in the middle of performing the learned sequence, and could possibly be preventing the principal neurons from initiating another routine until the current one was finished.

Graybiel's lab is now investigating further how the interaction between these two groups of neurons helps to encode habitual behavior in the striatum.